Up to date, tagatose has been prepared from galactose by chemical synthesis. According to the conventional method of chemical synthesis, galactose is isomerized in the presence of inorganic salt, mediated by metal hydroxide as a catalyst, to form the intermediate of the metal hydroxide-tagatose complex. The intermediate is neutralized by acid to produce tagatose. However, this conventional method of chemical synthesis is very complicated in its process and inefficient and generates industrial wastes, even if it might be economical and give the product with high yield.
Another method to produce tagatose is an enzyme mediated conversion, in which aldose or aldose derivatives are converted into ketose or ketose derivatives, followed by the conversion of galactose into tagatose using the enzyme. It is known that arabinose isomerase that has been used for the conversion of L-arabinose into L-ribulose can generate tagatose by using galactose as a substrate in vitro. Accordingly, many arabinose isomerases having different origins and manufacturing methods of tagatose from galactose using the same have been reported.
Isomerization of galactose to tagatose is a kind of reversible reaction, suggesting that the concentration of the reactant is balanced with the concentration of the product. This enzyme reaction is characterized by that the conversion rate relatively increases in proportion to the reaction temperature thermodynamically. Therefore, development of a novel enzyme stable at high temperature and the process of isomerization using the same are key factors for the enzyme mediated conversion using isomerase to produce tagatose.
The conventional arabinose isomerase originated from E. coli exhibited very low tagatose conversion rate from galactose, which was 25%, when the isomerization was carried out at 30° C. for 24 hours. Arabinose isomerase originated from the thermophilic microorganism Geobacillus stearothermophilus exhibited 46% of conversion rate at 60° C. which is a stable reaction condition have been reported. When hyperthermophilic arabinose isomerase originated from Thermotoga maritima was used for the isomerization reaction at 70° C. and 80° C., the conversion rate from galactose to tagatose was 50% and 56%, respectively (Oh, D. K., Kim, H. J., Ryu, S. A., Kim, P., 2001. Development of an immobilization method of I-arabinose isomerase for industrial production of tagatose. Biotechnol. Lett. 23, 1859-1862; Kim, H. J., Oh, D. K., 2005. Purification and characterization of an L-arabinose isomerase from an isolated strain of Geobacillus thermodenitrificans producing d-tagatose, J. Biotech. Nov 4; 120(2):162-73. Epub 2005 Aug. 9; Lee, D. W., Jang, H. J., Choe, E. A., Kim, B. C., Lee, S. J., Kim, S. B., Hong, Y. H., Pyun, Y. R., 2004. Characterization of a thermostable I-arabinose (d-galactose) isomerase from the hyperthermophilic eubacterium Thermotoga maritima. Appl. Environ. Microbiol. 70, 1397-1404.).
The application of thermophilic enzyme and the reaction at high temperature can increase the conversion rate from galactose to tagatose gradually. However, in general, browning of sugar solution at reaction temperature of 70° C. or up is dramatically increase in proportion to the temperature rise. In addition, the byproducts generated by such high temperature reaction have a bad influence on the purity of the final product and purification costs. Therefore, the reaction temperature rise is limited. So, the best applicable reaction temperature seems to be 70° C. and the maximum conversion rate of isomerization at that temperature is 56%. The higher conversion rate has not been obtained, so far.